Even if you are not competing, how can you "push yourself" without quantifying it somehow?
When I did my tri I knew I wasn't going to place well, but that was ok.. I set a personal goal that was achievable with hard work and dedicated training. Because of the clock, I was able to know if I met it or not and truly push myself to get there. You can't do that if you have to wait for obstacles and they don't time it, thats my big hang up on trying one of these.